Julian Astle writes about politics and public policy. He previously worked as director of CentreForum, the liberal think tank, and as Political Advisor to Paddy Ashdown when leader of the Liberal Democrats. He is @JulianAstle on Twitter.

The legacy of the SDP: we're all centrists now

Question: What have Andrew Adonis, Chris Huhne and Greg Clark got in common? Answer: Thirty years ago, they all joined the SDP.

Which is why, last night, we at CentreForum, the liberal think tank , invited these three men, now representing three different parties, to debate the legacy of the SDP at an event to mark the 30th anniversary of the party's formation.

Most of the discussion was of interest only to those involved at the time, so I'll spare you the details. But one thought occurred as I listened to the speakers: that the SDP may not have "broken the mould" of British politics as they set out to do, but they did succeed in creating a less polarised political culture which continues to punish any party that moves too far from the centre ground. Those who question this should consider the following:

At the 1992 election, Paddy Ashdown presented his party, the newly formed Liberal Democrats, as the only "non-socialist alternative to Thatcherism".

By the 1997 election, after three years of modernisation in the Labour party, there were two parties claiming to be a "non-socialist alternative to Thatcherism".

And by 2010, after four years of modernisation in the Conservative party, the electorate were asked to choose between three "non-socialist alternatives to Thatcherism".

So all those naive, middle class revolutionaries that flocked to the SDP – 60 per cent of whom had never belonged to a political party before – may not have achieved the breakthrough their 50+ per cent poll ratings once threatened. But they did make a difference. You can celebrate it or regret it, but the fact remains: we're all centrists now.